Hello! Welcome to TechSpot!
Definatley not a fad, however there is the xtreme too.
Extreme uses HT and dual core, both have 1mb cache for each core.
The extreme goes much higher fsb 1066, the D is 800.
What I don't like reading about them is it appears you need to run
the Intel® 955X Express chipset
Intel® 945P Express chipset
Intel® 945G Express chipset
Intel® E7230 Chipset
to get them to run right.